1. Field of the Invention
The present invention relates to a device, provided in an electronic still camera, to limit the maximum value of an exposure period.
2. Description of the Related Art
Conventionally, there is known an electronic still camera which can perform a long-term exposure which is 10 and a few seconds, for example, such as a bulb photographing operation. The electronic still camera is provided with an imaging device such as a CCD, in which an electronic charge, corresponding to a subject image formed on the light-receiving surface, is accumulated. The amount of the accumulated electronic charge is controlled by adjusting the opening degree of an aperture and shutter speed which is an exposure period.
On the other hand, in the imaging device, when the temperature becomes too high, the amount of dark current generated in the photo-diode of the imaging device increases, so that the influence of the dark current becomes excessive when the exposure period is long, causing the noise component generated in the photographed image to increase. Namely, in a photograph in which a long-time exposure is carried out, the influence of the dark current is large, and thus the quality of the photographed image is easily affected.
Therefore, an object of the present invention is to provide an exposure period control device, which reduces the influence of noise occurring in a photographed image due to dark current, so that the image quality is not affected.
Another object of the present invention is to provide an exposure period control device, which can inform the photographer that the influence of noise might occur in the photographed image.
According to the present invention, there is provided an exposure period control device for an electronic still camera, comprising a thermo sensor and a maximum exposure period control processor. The thermo sensor senses the temperature of an imaging device. The maximum exposure period control processor limits the maximum exposure period for an automatic exposure control to a value corresponding to the first temperature.
Further, according to the present invention, there is provided an exposure period control device for an electronic still camera, which can perform a long-time exposure, the exposure period control device comprising a temperature sensing processor and a warning processor. The temperature sensing processor senses the temperature of the environment where the electronic still camera is used. The warning processor obtains a maximum exposure period for long-time exposure, in accordance with the temperature, to output a warning signal corresponding to the maximum exposure period.